Showing papers on "Film temperature published in 2006"

Mixed convection boundary layers in the stagnation-point flow toward a stretching vertical sheet

Abstract: An analysis is made for the steady mixed convection boundary layer flow near the two-dimensional stagnation-point flow of an incompressible viscous fluid over a stretching vertical sheet in its own plane. The stretching velocity and the surface temperature are assumed to vary linearly with the distance from the stagnation-point. Two equal and opposite forces are impulsively applied along the x-axis so that the wall is stretched, keeping the origin fixed in a viscous fluid of constant ambient temperature. The transformed ordinary differential equations are solved numerically for some values of the parameters involved using a very efficient numerical scheme known as the Keller-box method. The features of the flow and heat transfer characteristics are analyzed and discussed in detail. Both cases of assisting and opposing flows are considered. It is observed that, for assisting flow, both the skin friction coefficient and the local Nusselt number increase as the buoyancy parameter increases, while only the local Nusselt number increases but the skin friction coefficient decreases as the Prandtl number increases. For opposing flow, both the skin friction coefficient and the local Nusselt number decrease as the buoyancy parameter increases, but both increase as Pr increases. Comparison with known results is excellent.

Similiarity solutions for the unsteady boundary layer flow and heat transfer due to a stretching sheet

Abstract: A similarity analysis is presented to investigate the unsteady boundary layers over a stretching sheet for special distributions of the stretching velocity and surface temperature or surface heat flux. The governing unsteady boundary layer equations are reduced to ordinary differential equations with two parameters, the Prandtl number and the unsteadiness parameter. These equations are solved numerically for some values of the governing parameters using the Keller-box method. Some flow and heat transfer characteristics are determined and discussed in detail.

Effect of viscous dissipation on heat transfer in a non-Newtonian liquid film over an unsteady stretching sheet

Abstract: Convective heat transfer of non-Newtonian fluids within a thin liquid film on an unsteady stretching sheet is investigated, taking into consideration the viscous dissipation effect. Results for the temperature distribution, the free-surface temperature, and the wall temperature gradient are illustrated at selected values of the unsteadiness parameter, the power-law index, and Eckert number for a wide range of the generalized Prandtl number, ranging from 0.001 to 1000. Also, new results for the velocity profiles, the free-surface velocity, and the wall shear stress are presented. The deviation from Newtonian behavior on the variation of the horizontal velocity component across the liquid film is observed more significant than that reported in the previous investigation. As compared to the case where viscous dissipation is neglected, the dimensionless fluid temperature is found to increase when the fluid is being heated but to decrease when the fluid is being cooled. For the fluid heating case, the dimensionless fluid temperature decreases monotonically in the vertical direction; while for the fluid cooling case it decreases rapidly at first, reaches a minimum value, and then increases more gradually to its free-surface value. The wall temperature gradient takes a higher value for a negative Eckert number but a lower value for a positive Eckert number, as compared to the case without viscous dissipation. The effects of positive or negative Eckert numbers on heat transfer are found to be more pronounced for higher generalized Prandtl numbers.

Effects of Reynolds and Prandtl Numbers on Heat Transfer Across a Square Cylinder in the Steady Flow Regime

Abstract: The effects of Reynolds and Prandtl numbers on the heat transfer characteristics of an isolated square cylinder have been investigated for the range of conditions 1 ≤ Re ≤ 45 and 0.7 ≤ Pr ≤ 4,000 (the maximum value of Peclet number being 4,000) in crossflow. Heat transfer correlations are obtained for the constant cylinder temperature and constant heat flux boundary conditions on a solid square cylinder in the steady flow regime. In addition, the variation of local Nusselt number on each face of the obstacle and representative isotherm plots are presented to elucidate the role of Prandtl number on heat transfer in the steady flow regime.

Second Law Analysis in Convective Heat and Mass Transfer

Abstract: This paper reports the numerical determination of the entropy generation due to heat transfer, mass transfer and fluid friction in steady state for laminar double diffusive convection, in an inclined enclosure with heat and mass diffusive walls, by solving numerically the mass, momentum, species conservation and energy balance equations, using a Control Volume Finite-Element Method. The influences of the inclination angle, the thermal Grashof number and the buoyancy ratio on total entropy generation were investigated. The irreversibilities localization due to heat transfer, mass transfer and fluid friction is discussed for three inclination angles at a fixed thermal Grashof number.

Effects of Reynolds and Prandtl numbers on the heat transfer across a square cylinder in the steady flow regime

Abstract: The effects of Reynolds and Prandtl numbers on the heat transfer characteristics of an isolated square cylinder have been investigated for the range of conditions 1≤Re≤45 and 0.7≤Pr≤4,000 (the maximum value of Peclet number being 4,000) in crossflow. Heat transfer correlations are obtained for the constant cylinder temperature and constant heat flux boundary conditions on a solid square cylinder in the steady flow regime. In addition, the variation of local Nusselt number on each face of the obstacle and representative isotherm plots are presented to elucidate the role of Prandtl number on heat transfer in the steady flow regime.

Height Effect on Heat-Transfer Characteristics of Aluminum-Foam Heat Sinks

Abstract: This study investigates and demonstrates the two conflicting effects of the height on the cooling performance of aluminum-foam heat sinks, under the impinging-jet flow condition. In addition, the nonlocal thermal equilibrium phenomena are also investigated. When the HID (the height to diameter ratio) of the aluminum-foam heat sinks is reduced from 0.92 to 0.15, the Nusselt number of aluminum-foam heat sinks is found to first increase and then decrease. The increase in the Nusselt number is caused by the increased percentage of the cooling air reaching the top surface of the waste-heat generation block, resulting from the reduced flow resistance. The decrease in the Nusselt number is mainly caused by the reduction in the heat-transfer area between the cooling air and the solid phase of the aluminum-foam heat sink. As the porosity and pore density decrease, the Nusselt number increases and the convective heat transfer is enhanced. The correlation between the Nusselt and Reynolds numbers for each of the 15 samples studied in this work is reported. For samples with a H/D> 0.31, the temperature difference between the solid and gas phases of aluminum-foam heat sinks decreases with the increase of the distance from the heated surface. The non-local thermal equilibrium regime is observed to exist at low Reynolds number and small dimensionless height. On the other hand, for samples with a HID ≤ 0.31, the temperature difference first increases and then decreases with the increase of the distance from the heated surface; the maximum temperature difference is located at z/H≒0.25 and is independent of the Reynolds number.

Fluid Flow and Heat Transfer in Power-Law Fluids Across Circular Cylinders: Analytical Study

Abstract: An integral approach of the boundary layer analysis is employed for the modeling of fluid flow around and heat transfer from infinite circular cylinders in power-law fluids. The Von Karman-Pohlhausen method is used to solve the momentum integral equation whereas the energy integral equation is solved for both isothermal and isoflux boundary conditions. A fourth-order velocity profile in the hydrodynamic boundary layer and a third-order temperature profile in the thermal boundary layer are used to solve both integral equations. Closed form expressions are obtained for the drag and heat transfer coefficients that can be used for a wide range of the power-law index, and generalized Reynolds and Prandtl numbers. It is found that pseudoplastic fluids offer less skin friction and higher heat transfer coefficients than dilatant fluids. As a result, the drag coefficients decrease and the heat transfer increases with the decrease in power-law index. Comparison of the analytical models with available experimental/numerical data proves the applicability of the integral approach for power-law fluids. DOI: 10.1115/1.2241747

Natural Convection in a Square Cavity with Spatial Side-Wall Temperature Variation

Abstract: Laminar natural convection in a two-dimensional square cavity filled with a pure air (Pr = 0.71) is studied numerically in the present article with nonuniform side-wall temperature. The heated vertical wall is assumed to have spatial sinusoidal temperature variations about a constant mean value, which is higher than the cold side-wall temperature, while the top and the bottom walls are adiabatic. A finite-volume method is used to solve numerically the nondimensional governing equations in the vorticity–stream function formulation. The effects of the amplitude and the wave number of the heated side-wall temperature variation on the natural convection in the cavity are investigated. It is found that the average Nusselt number varies based on the hot-wall temperature. It increases with an increase in the amplitude, while the maximum average Nusselt number occurs at the wave number of k = 0.7 for Rayleigh number range 103 ≤ Ra ≤ 106. It is found that the values of maximum fluid circulation occur at a...